A variety of 27 to 32 nm viruses have been shown to be etiologically associated with epidemic nonbacterial gastroenteritis. The Norwalk virus is the most thoroughly studied member of this group. Seroepidemiologic studies using a sensitive solid phase RIA indicate: (1) Norwalk virus has a world wide distribution, (2) Norwalk infection is responsible for at least 1/3rd of outbreaks of nonbacterial gastroenteritis in the U.S., and (3) the majority of outbreaks are waterborne but some are associated with contaminated food. Biophysical characterization of Norwalk virus indicates that its proteins resemble those of caliciviruses. Recently a serologically distinct 27 nm gastroenteritis virus, the Marin County virus, was successfully transmitted to adult volunteers. One volunteer shed large amounts of virus. Marin County virus shed by the volunteer was purified by ultracentrifugation techniques and used to make hyperimmune antisera which was used to develop a sensitive and specific RIA. Purified virus was characterized and shown to have 1 or 2 structural proteins (30 Kd). Monoclonal antibodies to purified Marin County virus are currently being isolated.